The present invention relates to a method and apparatus for operating an electrochemical fuel cell with periodic reactant starvation at an electrode. More particularly, the method comprises periodically momentarily fuel starving at least a portion of the anode of an operational fuel cell or periodically momentarily oxidant starving at least a portion of the cathode of an operational fuel cell or both. The method and apparatus may be used to improve fuel cell performance without suspending the generation of power by the fuel cell.
Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to produce electric power and reaction products. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) comprising a solid polymer electrolyte or ion-exchange membrane disposed between two porous electrically conductive electrode layers. The anode and cathode each comprise electrocatalyst, which is typically disposed at the membrane/electrode layer interface, to induce the desired electrochemical reaction.
At the anode, the fuel moves through the porous anode layer and is oxidized at the anode electrocatalyst to produce protons and electrons. The protons migrate through the ion exchange membrane towards the cathode. On the other side of the membrane, the oxidant moves through the porous cathode and reacts with the protons at the cathode electrocatalyst. The electrons travel from the anode to the cathode through an external circuit, producing an electrical current.
Electrochemical fuel cells can operate using various reactants. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be substantially pure oxygen or a dilute stream such as air containing oxygen.
The fuel stream may contain impurities that do not contribute to, and may actually inhibit, the desired electrochemical reaction at the anode. These impurities may, for example, originate from the fuel stream supply itself, or may be generated in situ in the fuel cell, for example, as intermediate species during the fuel cell reactions. Further, impurities may enter the fuel stream from elsewhere in the system. In a like manner, although less commonly, the oxidant stream may also contain impurities which may inhibit the desired electrochemical reaction at the cathode. Again, impurities may originate within the cathode stream, may be generated in situ, or may originate elsewhere in the system (e.g., fuel stream species may crossover from the anode to the cathode side of a solid polymer fuel cell by diffusion through the membrane electrolyte). Some of these impurities may be chemically adsorbed or physically deposited on the surface of the electrode electrocatalyst, blocking the active electrocatalyst sites and preventing these portions of the electrode electrocatalyst from inducing the desired electrochemical fuel oxidation or oxidant reduction reactions. Such impurities are known as electrocatalyst xe2x80x9cpoisonsxe2x80x9d and their effect on electrochemical fuel cells is known as xe2x80x9celectrocatalyst poisoningxe2x80x9d. Electrocatalyst poisoning thus results in reduced fuel cell performance, where fuel cell performance is defined as the voltage output from the cell for a given current density. Higher performance is associated with higher voltage for a given current density or higher current for a given voltage.
In the absence of countermeasures, the adsorption or deposition of electrocatalyst poisons may be cumulative, so even minute concentrations of poisons in a fuel stream, may for instance, over time, result in a degree of electrocatalyst poisoning which is detrimental to fuel cell performance.
Reformate streams derived from hydrocarbons or oxygenated hydrocarbons typically contain a high concentration of hydrogen fuel, but typically also contain electrocatalyst poisons such as carbon monoxide. To reduce the effects of anode electrocatalyst poisoning, it is known to pre-treat the fuel supply stream prior to directing it to the fuel cell. For example, pre-treatment methods may employ catalytic or other methods to convert carbon monoxide to carbon dioxide. However, known pretreatment methods for reformate streams cannot efficiently remove 100% of the carbon monoxide. Even trace amounts less than 10 ppm can eventually result in electrocatalyst poisoning which causes a reduction in fuel cell performance.
Substances other than carbon monoxide are also known to poison fuel cell electrocatalysts. Depending on the type of fuel and the fuel processing methods, impurities in the fuel stream may be present in quantities sufficient to poison the electrocatalyst and reduce fuel cell performance. Fuel cell components and other fluid streams in the fuel cell system may also be a source of impurities that may result in poisoning of the electrocatalyst on either or both electrodes. For example, fuel cell separator plates are commonly made from graphite. Organic impurities in the graphite may leach out and poison the electrocatalyst. Other poisons may be generated by the reaction of substances in the reactant streams with the fuel cell component materials. Alternatively, substances present in one reactant stream may diffuse through the electrolyte and thus crossover from one electrode to the other. The crossover substance may be acceptable at the first electrode but may represent a poison at the other (for instance, in principle, methanol crossover from the anode to the cathode in a direct methanol fuel cell can depolarize or otherwise adversely affect the cathode).
What constitutes a poison may depend on the nature of the fuel cell. For example, whereas methanol is the fuel in a direct methanol fuel cell, in a hydrogen fuel cell operating on a methanol reformate stream, traces of unreformed methanol can be detrimental to the electrocatalyst performance at the anode.
Conventional methods for addressing the problem of electrode electrocatalyst poisoning include purging the electrode with an inert gas such as nitrogen. However, such purging methods involve suspending the generation of power by the fuel cell. A secondary power source is therefore needed to provide power while the fuel cell electrode is being purged.
Another approach for removing carbon monoxide or other poisons from an electrocatalyst comprises introducing a xe2x80x9ccleanxe2x80x9d reactant stream containing substantially no poisons to a poisoned fuel cell electrode. Where adsorption is reversible, an equilibrium process induced by introducing a clean reactant stream results in some rejuvenation of the electrocatalyst. However, a disadvantage of this approach is that it is generally not effective against irreversibly adsorbed poisons. Furthermore, the recovery of the electrode electrocatalyst by such an equilibrium process can be very slow, during which time the fuel cell is not able to operate at full capacity.
Another approach to counteract carbon monoxide electrocatalyst poisoning at the anode is to continuously introduce a low concentration of oxygen into the fuel stream upstream of the fuel cell, as disclosed in Gottesfeld U.S. Pat. No. 4,910,099. However, there are several disadvantages to Gottesfeld""s method which influence fuel cell performance and efficiency. For example, an oxygen bleed results in parasitic losses, undesirable localized exothermic reactions at the anode, and dilution of the fuel stream.
U.S. patent application Ser. No. 08/998,133 filed Dec. 23, 1997, now U.S. Pat. No. 6,096,448, entitled xe2x80x9cMethod and Apparatus for Operating an Electrochemical Fuel Cell With Periodic Fuel Starvation At The Anodexe2x80x9d is incorporated herein by reference in its entirety.
It is apparent from the prior art that there is a need for an improved method and apparatus for rejuvenating a fuel cell electrode electrocatalyst by removing poisons therefrom, which does not involve suspending the availability of the fuel cell to generate power.
A fuel cell is operated to produce electrical power for an electrical load by supplying an oxidant stream to the fuel cell cathode, and a fuel stream to the fuel cell anode. The present method comprises periodically oxidant starving at least a portion of the cathode, while continuing to produce electrical power from the fuel cell. Typically, when the method is applied, the fuel cell performance after the starvation is improved relative to the performance just prior to the starvation. A performance improvement may result for various reasons. For instance, the production of water at the cathode may be briefly reduced thereby improving water management in the cell. Or, an improvement may result from the removal of electrocatalyst poisons, which is facilitated as the cathode potential decreases as occurs during oxidant starvation at the cathode. Oxidant starvation may have other benefits or effects at the electrodes, for example with regards to any heat generated as a result of the starvation. For example, this heat may be effective in regenerating the interfaces within membrane electrode assemblies after prolonged fuel cell operation.
The fuel cell is preferably a solid polymer fuel cell. The fuel and oxidant streams may be gaseous or liquid. The fuel cell may, for example, be a direct methanol fuel cell.
In a first embodiment, the method for oxidant starving at least a portion of the fuel cell cathode comprises periodically interrupting the supply of the oxidant stream to the fuel cell cathode. This can be accomplished, for example, by adjusting a valve upstream of the fuel cell cathode, stopping an oxidant supply compressor, or diverting the oxidant supply stream away from the fuel cell cathode.
Where the fuel cell is one of a plurality of fuel cells, for example, arranged in a fuel cell stack, the method preferably comprises preventing the simultaneous interruption of the supply of oxidant to each cathode of the plurality of fuel cells. This reduces the magnitude of fluctuations in electrical power output from the stack.
The first embodiment of the method may further comprise closing a valve downstream of the fuel cell cathode substantially simultaneously with the interruption of supply of the oxidant stream to prevent the oxidant stream from being exhausted from the fuel cell.
In a second embodiment, the method for oxidant starving at least a portion of the fuel cell cathode comprises periodically introducing pulses of a substantially oxidant-free fluid into the oxidant stream upstream of the fuel cell cathode. The substantially oxidant-free fluid moves through the cathode flow field, thereby oxidant starving successive portions of the cathode.
The substantially oxidant-free fluid may contain some oxidant, provided the oxidant concentration is sufficiently low to induce oxidant starvation of portions of the cathode with which the fluid is in is contact, and thereby give the desired recovery in performance of the fuel cell. Preferably the substantially oxidant-free fluid contains essentially no oxidant and is substantially unreactive at the fuel cell cathode, for example, nitrogen, argon, and helium. Alternatively, the substantially oxidant-free fluid may comprise quantities of components that participate in and enhance the reactions at the cathode but are not themselves detrimental to fuel cell performance.
The oxidant and the substantially oxidant-free fluid may both be in the same phase or different phases. In particular, the oxidant stream may be a gas stream and the substantially oxidant-free fluid may also be gaseous.
The method may further comprise introducing a substantially oxidant-free fluid pulse that is cooler than the internal operating temperature of the fuel cell. In this embodiment, the substantially oxidant-free fluid may act as a coolant for the fuel cell. Similarly, substantially oxidant-free fluid could be introduced at a temperature higher than the operating temperature of the fuel cell, in situations where it is desirable to raise the fuel cell operating temperature.
The method for introducing the substantially oxidant-free pulse may comprise the steps of periodically closing an oxidant supply valve to stop the flow of the oxidant stream upstream of the fuel cell and simultaneously opening an interrupt valve to introduce a pulse of a substantially oxidant-free fluid stream into the oxidant stream. In a variation on this embodiment, the oxidant supply stream is maintained at a lower pressure than the substantially oxidant-free fluid stream, and the method of introducing the substantially oxidant-free fluid comprises periodically opening an interrupt valve to introduce a pulse of a substantially oxidant-free fluid stream into the oxidant stream.
In a third embodiment, the method for oxidant starving at least a portion of the fuel cell cathode comprises periodically connecting a transient electrical load to draw electrical power from the fuel cell. Preferably, the rate of supply of the oxidant stream to the fuel cell cathode is not increased in response to the connection of the transient load, so that oxidant in the fuel cell is consumed at a faster rate than it is supplied and at least a portion of the cathode becomes oxidant starved. The transient electrical load may comprise a capacitor which may be used to release an electrical charge, for example, when the power demand from the electrical load exceeds the power output of the fuel cell during times when the fuel cell is undergoing rejuvenation.
Where the fuel cell is one of a plurality of fuel cells, for example, arranged in a fuel cell stack, preferably the periodic connection of the transient load is not connected to draw electrical power from all the fuel cells simultaneously.
In the embodiments described above the oxidant starvation may be induced at regular time intervals, for example, by interrupting the oxidant supply, introducing substantially oxidant-free pulses or connecting a transient load at regular time intervals. Alternatively, the method may comprise monitoring an operational parameter (e.g., cell voltage) of the fuel cell and adjusting the frequency with which the oxidant starvation is induced in response to the value of the monitored parameter. Similarly, the duration of the oxidant starvation may be fixed or varied, for example in response to a monitored operational parameter.
One or both the duration and frequency of the periodic interruptions may be selected as a function of the concentration of the catalyst poisoning species in the oxidant stream.
In the above embodiments, it is generally preferred that cell reversal is avoided. However, an embodiment of the method for operating a fuel cell assembly comprising a plurality of fuel cells, may comprise periodically oxidant starving at least one, but not all, of the fuel cell cathodes such that a cell reversal occurs, while continuing to generate electrical power from the remaining cells. Preferably however, the oxidant starvation is limited so that the cell reversal is not prolonged.
In a first embodiment, a fuel cell apparatus comprises an oxidant supply system for directing an oxidant stream to a cathode of the fuel cell, a flow controller for periodically interrupting the supply of the oxidant stream to the cathode, and an actuator associated with the flow controller for controlling the frequency and duration of the interruptions.
The flow controller may comprise an oxidant supply valve located upstream of the cathode, and the actuator is preferably connected to periodically partially or preferably fully close the oxidant supply valve to interrupt the oxidant supply to the cathode. The fuel cell apparatus may further comprise an oxidant exhaust stream valve located downstream of the cathode which is activated by the actuator (or a second actuator activated in coordination with the first actuator) to open and close in coordination with the oxidant supply valve.
The oxidant supply system may comprise a compressor for directing an oxidant stream to the cathode. In this embodiment, the actuator may, for example, be connected to periodically deactivate the compressor and thereby interrupt the oxidant supply to the cathode. An oxidant exhaust stream valve located downstream of the cathode may be activated by the actuator in coordination with the compressor activation to close the valve when the compressor is periodically deactivated, and open the valve when the compressor is re-activated.
The flow controller may comprise a diverter located upstream of the cathode for diverting the oxidant stream away from the cathode. The diverter may be periodically actuated by the actuator.
A sensor may be employed that responds to fuel cell performance (e.g., voltage) or, where applicable, that detects the concentration of catalyst poisons in the oxidant stream. The sensor may provide an output signal to the actuator that adjusts the frequency and/or duration of the interruptions in response to the sensor output signal.
The fuel cell may comprise a plurality of independent oxidant flow field channels for directing the oxidant stream in contact with the cathode. Each one of the flow field channels directs the oxidant stream to a discrete region of the cathode and the supply of the oxidant stream to each one of the regions can be controlled independently from the supply of the oxidant stream to other ones of the regions. In this embodiment, selected regions of the cathode can be oxidant starved while other regions continue to contribute to the fuel cell power output.
In a second embodiment, a fuel cell apparatus comprises a oxidant supply system for directing an oxidant stream to a cathode of the fuel cell, a source of a substantially oxidant-free fluid, and a flow controller for periodically introducing pulses of the substantially oxidant-free fluid into the oxidant stream upstream of the fuel cell cathode. The flow controller may comprise an interrupt valve for controlling the introduction of the substantially oxidant-free fluid stream into the oxidant stream.
In a third embodiment, a fuel cell apparatus comprises a transient electrical load that is selectively electrically connected to draw electrical power from the fuel cell. A switch periodically electrically connects the transient electrical load to draw electrical power from the fuel cell. An actuator associated with the switch controls the frequency and duration of the electrical connection. The transient load may comprise a capacitor for storing an electrical charge that can be released to the electrical load.
It may be advantageous to perform periodic fuel starvation as well as oxidant starvation, either simultaneously or in a preferred sequence. The application of a transient load may be the simplest way to implement simultaneous fuel and oxidant starvation.
The embodiments described above may be used to improve fuel cell performance and increase the service life of an electrochemical fuel cell.